Those applications don't support color fonts. The font should be for Firefox's use only, precisely for that reason. For example, on Windows, it can be found under <installation folder>\fonts\EmojiOneMozilla.ttf so other applications don't see it.

@morat - It's visible for Firefox and SeaMonkey - doesn't show in direct look at Thunderbird directories or an in depth search.However - Last few emails there was a happy face which showed after the usual colon right parenthesis.